This invention is related to the field of integrated optics (that is, integrated photonics). In particular, the present devices are photonic input/output (I/O) ports designed for effective coupling of optical signals from a planar photonic circuit to an external optical fiber, or vice versa. Moreover, the present I/O ports enable the implementation of balanced photonic circuits for polarization independent operation.
Effective optical (that is, photonic) telecommunication systems require high-performance, low-cost photonic devices. Such a requirement has motivated development of integrated photonic circuits that are planar or substantially planar. Those circuits interface with other devices and system components using input/output (I/O) ports, which may be referred to as couplers or grating couplers and which typically optically connect planar or substantially planar circuits to cylindrical optical fibers. Such I/O ports can act as input ports, output ports, or bi-directional ports. As used herein, the terms input port(s), output port(s), bi-directional port(s) and I/O port(s) may be used interchangeably. In other words, unless otherwise specified, each of those terms contemplates and includes all of those terms.
In planar or substantially planar photonic circuits, coupling to or from an optical fiber is commonly achieved in an axial geometry arrangement using a system of lenses (FIG. 1(a)), or by directly attaching the fiber to the planar or substantially planar photonic circuit (FIG. 1(b)). The problems with such approaches include the need for photonic circuit surfaces of high quality (that is, highly smooth, planar or substantially planar surfaces, which may be prepared by cleaving and/or polishing and through which a photonic signal may pass), and the need for highly accurate mechanical alignment of micro-photonic elements. In addition, to be effective, many planar or substantially planar photonic circuits are required to be polarization independent (that is, to operate substantially the same way for any input polarization). Consistently achieving polarization independence in effective axially-coupled planar or substantially planar circuits has proven to be generally difficult and, in some cases, has resulted in I/O ports that compromise a circuit""s overall performance or flexibility. Moreover, devices, such as I/O ports, fabricated on the same wafer cannot be properly tested until after separation into individual elements. Such testing constraints have further complicated efforts to commercialize effective telecommunication systems.
The present I/O ports can be effectively incorporated into planar or substantially planar photonic circuits, and the present I/O ports effectively couple light to optical fibers. The present I/O ports can effectively couple light at normal or near-normal incidence to the plane of the photonic circuit. As used herein, the term xe2x80x9cnear-normalxe2x80x9d shall mean and include angles up to approximately 30xc2x0 away from normal (that is angles ranging from approximately xe2x88x9230xc2x0 to approximately +30xc2x0), and the term xe2x80x9coff-normalxe2x80x9d shall mean and include all xe2x80x9cnear-normalxe2x80x9d angles except those angles equal to approximately 0xc2x0. In near normal geometry, light from an optical fiber is shone either indirectly, using a system of lenses (as shown in FIG. 2(a)), or directly (as shown in FIG. 2(b)) onto the input port located on the top (or bottom) surface of the planar or substantially planar photonic circuit.
In effective optical telecommunication systems, particularly those employing dense wavelength-division multiplexing (DWDM), I/O ports are usually operable over a wide band of input frequencies and, thus, over a wide band of input wavelengths. Current commercially available optical telecommunications systems employ wavelengths from approximately 1525 to approximately 1565 nm, a range known as the C-band, and wavelengths from approximately 1565 to approximately 1620 nm, a range known as the L-band. It is therefore important to control (for example, to maximize), the operational bandwidth of an I/O port. As used herein, the term xe2x80x9ccontrolxe2x80x9d shall mean and include minimize, maximize, reduce, increase and/or achieve a desired or effective level or range, unless otherwise specified.
It is also important to control (for example, to maximize) coupling efficiency, with coupling efficiency being the fraction of light incident upon the I/O port that is transferred into the coupled circuit. Similarly, controlled insertion loss is desired. Insertion loss, expressed in decibels (dB), is defined as ten times the base ten logarithm of the inverse of the coupling efficiency.
Prior work in connection with or relating to I/O ports featuring the geometry of FIG. 2 has been conducted. Such prior I/O ports have been used in connection with normal and near normal incidence coupling and typically comprise an optical waveguide and one-dimensional or approximately one-dimensional grating, which is a periodic arrangement of grooves or straight lines. The grating grooves or lines serve as optical scattering elements for incident light, and are arranged to direct near-normal incident light into the plane of the device in a coupling region.
Prior work in connection with or relating to I/O ports featuring the geometry of FIG. 2 can be distinguished from the present I/O ports by, for example, considering the index contrast in a coupling region, xcex94n. xcex94n is defined as the difference between the maximum refractive index and the minimum refractive index of the respective constituent materials in the coupling region (that is, the respective constituent materials comprising an optical scattering element, which is defined below). Those constituent materials may, as explained below, be air and the material of which the coupling region is made. Prior work on near-normal-incidence couplers has concerned low index contrast gratings in low index contrast waveguides. Such prior work has suffered from limited coupling bandwidth, insertion loss and/or sensitivity to angular misalignment.
It is desirable to achieve effective operation of a planar or substantially planar photonic circuit with the simple direct fiber attachment of FIG. 2(b). For a conventional single-mode optical fiber (that is, an optical fiber that supports only one propagating mode at the operating wavelength), such as the fiber illustrated in FIG. 2(b), the spatial profile of the optical mode can be considered Gaussian. The mode field diameter of the optical mode, which diameter is defined as the full width at the 1/e2 intensity points, is typically on the order of 10 xcexcm. Prior planar or substantially planar photonic circuits suffer from higher insertion loss with such small mode field diameters, and, accordingly, such circuits usually require beam expanding optics in order to adapt the mode field diameter of the fiber to the larger mode field diameter characteristic of prior I/O ports.
Prior I/O ports designed to couple light at near-normal incidence typically suffer from excessive polarization dependence. In other words, certain optical performance specifications for a photonic circuit (such as, for example, the insertion loss for the circuit), depend upon the polarization of the input light. More specifically, prior work shows that prior near-normal incidence I/O ports suffer from polarization-dependent loss (PDL), which is defined as the maximum amount of insertion loss variation observed for the photonic circuit while varying the input light over all possible states of polarization.
A number of suggested solutions to the above problems, which suggested solutions are taught by prior work, involve complex systems of micro-optical elements or complex fabrication sequences. Such suggested solutions are unacceptable for one or more reasons, including, the tendency for such micro-optical elements to move (physically) over time, the cost of assembling those complex systems and the lower yields typically attributable to complex fabrication sequences.
Another important aspect of the design of an effective I/O port is its compatibility with in-plane waveguide interconnects of the planar photonic integrated circuit. It is preferable to use single mode waveguides in planar photonic integrated circuits. In many cases, the typical transverse mode profile of guided light is on the order of 1 xcexcm. As mentioned above, the typical mode field diameter exiting from a single mode optical fiber is on the order of 10 xcexcm. Therefore, to achieve meaningful effectiveness in an I/O port, an additional mode-size converting optical element is needed. One such appropriate element is a planar waveguide lens.